This invention relates to imaging systems employing radio frequency (RF) coils for RF field generation, and more particularly to RF coils for use in such systems which coils facilitate higher frequency, higher efficiency, higher energy operation, permit use of larger coils, facilitate flexibility in coil design to accommodate a variety of applications and provide enhanced signal-to-noise performance so as to achieve among other things improved MRI, fMRI and MR spectroscopic imaging, all the above being achieved without significant increase in cost. The invention applies similarly to EPR or ESR.
Nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI), functional MRI (fMRI), electron spin resonance (ESR) or electron paramagnetic resonance (EPR) and other imaging techniques using RF field generating coils are finding increasing utility in applications involving imaging of various parts of the human body, of other organisms, whether living or dead, and of other materials or objects requiring imaging or spectroscopy. For purposes of this application, RF shall be considered to include frequencies from approximately 1 MHz to 100 GHZ, the upper ranges of which are considered to be microwaves. While existing such systems are adequate for many applications, there is often a need for higher signal-to-noise and improved spectral resolution in such imaging so as to permit higher spatial resolution, higher image contrast, and faster imaging speed. In fMRI applications for example, where multiple images may be taken over time and a difference image generated to permit visualization of small changes in blood oxygen use over time in the body being imaged, differences between successive images may be very small, requiring high signal-to-noise to permit detection. A major limitation to higher resolution, and/or faster imaging is an insufficient signal to noise ratio. If the image signal intensity is below the noise level, an image can not be made. It is therefore important in high resolution systems to design an RF coil to maximize signal and to minimize noise. The RF coil of such a system should also be designed to minimize eddy currents propagating therein which are induced by time transient currents in gradient coils or by other causes.
The signal-to-noise ratio (SNR) and spectral resolution are increased by increasing the magnetic field strength of the system, generally expressed in tesla (T). The SNR benefits of image speed, spatial resolution, and contrast are also increased with the magnetic field strength. However, the frequency of which the nuclei of atoms in the body resonates varies as a function of the applied magnetic field, with each atomic species having a unique magnetic field dependent resonant frequency referred to as the Larmor frequency. For the human body which is composed primarily of hydrogen atoms in water, fat and muscle tissue, these hydrogen nuclear (proton) frequencies are approximately 64 MHZ for a field strength of 1.5 T, 170 MHZ (4 T), 175 MHZ (4.1 T), 300 MHZ (7 T), 340 MHZ (8 T) and 400 MHZ (9.4 T). Other species of atomic nuclei in a body would resonate at other frequencies for a given field strength. However, while conventional birdcage coils in existing MRI and related systems might resonate at a frequency of 170 MHZ (4 T) for example, the conventional birdcage coil with lumped elements (reactance) will operate very inefficiently, radiating much of its energy like an antenna, rather than conserving its energy like a xe2x80x9ccoilxe2x80x9d. At higher frequencies still, such lumped element coils of human head or body dimensions will not reach the Larmor resonant frequency required, limiting the magnetic field strength at which such MRI or EPR systems can operate. Further, since frequency is a function of the electrical path lengths (measured in wavelengths) in the RF coil, higher frequency, and thus higher field strength operation, has been previously achievable only with very small coils which are not always useful for imaging a human being or other larger objects. A need therefore exists for an RF coil design which provides short electrical path lengths and shields against radiative losses, while still permitting an RF coil to be constructed with physical dimensions sufficient to image a human body and/or other larger objects with high frequency RF energy, thus permitting high field strength operation. It is also desirable to be able to tune each path of an RF coil to a precise resonant frequency, to be able to provide two or more resonant frequencies for different paths on the coil, and to be able to easily adjust/retune the resonant frequency of a path or paths.
Still another potential problem in operating these imaging systems, especially at high fields, is in driving the RF coil in a manner so as to achieve a homogeneous RF field, even when a body is positioned in the field, or to achieve some other desired field profile. Many factors influence field profile or contours including the manner in which the coils are driven, the geometric and frequency dependent electrical properties of the anatomy or object, and the frequency dependent properties of the coil circuits. Techniques for controlling these and other factors to achieve a homogenous or other desired field profile are therefore desirable. Also, while in many systems the same coil is used for both the transmitting of RF energy and the receiving thereof, the coils being switched between transmit and receive circuitry, there are many applications where the homogeneous excitation of NMR signal is achieved with a large volume coil and a small local receive coil having very short path lengths is used for achieving high SNR operation, such local receive coil being placed as close to the region of the body being imaged as possible. However, having both the large transmit coil and the local RF receive coil tuned to the same frequency results in the coils being destructively coupled (by Lenz""s Law for example), this defeating enhanced operation from the local receive coil. It is therefore desirable to be able to quickly detune the large RF transmit coil during a receive operation by a local RF receive coil and vice versa. Improved ways of achieving this objective, particularly in an RF coil providing the characteristics previously indicated, are therefore desirable.
Finally, some of the advantages of having a local receive coil, and in particular the ability to place the RF coil closely adjacent to a region where imaging is desired, could be achieved if the RF coil were designed so as to localize both the transmission and reception of RF energy. While coils adapted for performing this function in certain specialized situations have existed in the past, a more general purpose design for RF coils to facilitate their use in producing localized RF fields and the localized reception of RF (NMR) signal, is desirable in order to achieve the enhanced SNR benefits of higher image signal, resolution, speed and contrast.
While some of the advantages indicated above are achieved by distributed impedance RF coils disclosed in U.S. Pat. Nos. 5,557,247 and 5,744,957, which patents have the same inventor as this invention, the systems taught in these patents, and in particular the RF coils thereof, do not provide optimum performance in all situations, and improvements are possible on various aspects of these RF coils, including eddy current suppression, design of the coil for optimum positioning in a greater number of cases, improved control of field profile, improved tuning options and improved detuning in situations where the use of two or more coils is desired.
In accordance with the above, this invention provides an RF coil for use in an imaging system, which coil has a cavity formed as a conductive enclosure in which resonant field can be excited, the enclosure being formed at least in part of an electrical conductor patterned to form RF conductive paths around the cavity. At least one tuning mechanism may be provided which determines a resonant frequency or frequencies for such paths. The tuning mechanism may be fixed, resulting in a preselected resonant frequency for the path, or variable to provide a tunable resonant frequency or frequencies. The tuning mechanism may reactively adjust the electrical length of each path to tune the path. The path reactance may also be adjusted to achieve a selected field profile for the coil. The tuning mechanism may tune all the paths to the same resonance frequency or may selectively tune the paths to resonate at two or more different frequencies. In particular, alternate ones of the paths may be tuned to resonate at a first frequency and the remaining ones of the paths tuned to resonate at a second frequency.
The coil may also include a dielectric which at least substantially fills the cavity. the thickness of the conductor for at least selected portions of the enclosure may be substantially greater than one skin depth at the resonant frequency, the dielectric filling the cavity having a dielectric constant different from that of air. This results in signals of different frequencies propagating on the outer and inner surfaces of the conductor.
Each of the N paths on the coil may have at least one non-conductive gap formed therein, and the tuning mechanism may include a reactance and/or an impedance across at least selected ones of said gaps. The reactance/impedance for at least selected ones of the gaps may be variable to control the resonant frequency for the corresponding path. The reactance for at least some embodiments includes a capacitor, the capacitance of which may be varied and/or an inductor the inductance of which may be varied. The variable impedance and/or reactance may be controlled to tune, retune and/or detune the path in which it is located. Where the enclosure is formed of an outer wall, inner wall, and side walls, end conductive lands may be formed on each of the walls, with corresponding lands on each wall being connected to form the paths and the gaps being formed in the conductor for each of the paths for at least one of the walls. For some embodiments, the gaps are formed in the outer wall conductor for each path.
The resonant frequency of the paths may be determined by distributed capacitance and distributed inductance for the path, the distributed capacitance being determined by the area of the electrical conductor for each path, a dielectric fill material for the cavity and/or the dimensions of the dielectric fill material. The electrical conductor forming each path may be a thin foil, the distributed inductance for the path being a function of the path length. At least one reactance component may be provided in at least selected ones of the paths, the reactance component being either distributed or lumped. A distributed or discrete reactance may be selected to achieve a desired resonant frequency for the paths. The paths have a cumulative reactance which includes at least in part the distributed capacitance/inductance, the cumulative reactance for the paths being tuned to result in D different resonant frequencies for the coil, every Dth path symmetrically spaced around the coil being tuned to the same frequency.
The coil may include a circuit which applies RF signal to and/or receives RF signal from M selectively spaced ones of said paths, where M is an integer and 1xe2x89xa6Mxe2x89xa6N. The RF signals may be phase shifted corresponding to a phase shift for the corresponding paths to provide circular or other polarization for the coil. Each RF signal is preferably reactively coupled to the corresponding path, the coupling reactance for each path being variable for some embodiments to independently match/tune the path. In particular, the coupling reactances may be impedance matched to different loading conditions for the coil. For some embodiments, the RF coil may be used to transmit and/or receive RF signals, but not both simultaneously, and includes a detuning mechanism for the paths, the detuning mechanism being operative when the RF coil is not in the one of the transmit/receive modes for which it is being used. The detuning mechanism may include a mechanism for altering the path length and/or impedance for each path to be detuned, and in particular may include for some embodiments a PIN diode circuit for each path which facilitates rapid switching to a changed impedance state sufficient to effect the path detuning. Alternatively, the RF drive signals may be phase-shifted corresponding to the phase shift for the paths to which they are applied to provide circular polarization for the coil, the detuning mechanism including circuitry which reverses the phase of the RF drive signals.
For an enclosure which is formed of an outer wall, inner wall, and side walls, within conductive lands being formed for each wall, corresponding lands on each wall being connected to form the paths, at least the outer wall may have two conductive layers separated by a dielectric, the two conductive layers each being slotted to form a pattern of lands, with slots on each layer being overlaid by lands of the adjacent layer. The degree of overlap for the lands of said layers is at least one factor controlling coil resonant frequency.
At least one of the side walls may also have an aperture through substantially the center thereof through which a body to be analyzed may be passed to an area inside the inner wall, the conductive layer on the inner wall being patterned to provide a selected magnetic flow pattern in said aperture. One of the side walls may also be closed, the closed side wall being slotted to form a land pattern covering at least most of the wall.
The imaging system may also have at least one gradient coil which induces low frequency eddy currents in the RF coil, the slotting on at least the outer wall and side walls resulting in the breaking up of and substantial attenuation of such eddy currents without substantial attenuation of RF currents and fields. The electrical conductor for at least the outer wall and side walls may be a conductive layer which is thin enough to attenuate low frequency eddy currents while still conducting RF currents. For such embodiment, the conductor layer has a thickness substantially equal to one skin depth at the resonance frequency to which the coil is tuned, which thickness is substantially equal to approximately 5 microns for an illustrative embodiment.
For some embodiments, each of the paths has at least one circumferential/azimuthal slot formed therein to break the path into smaller paths. A fixed, variable and/or switched reactive coupling and/or an impedance coupling may be connected across each of the circumferential slots. Where a reactive coupling is utilized, such coupling is a capacitive coupling for illustrative embodiments.
An RF drive signal input is provided to at least one of the paths, the path inductively coupling an RF drive signal and a path to adjacent paths.
The dielectric material filling the cavity may provide a selected path capacitance, and thus a selected resonant frequency. A mechanism may be provided for controlling the dielectric fill of the cavity and thus the resonant frequency of the coil.
The electrical conductor may be patterned to form N conductive lands for the enclosure, each of a selected width, and the number N of conductive paths and the width of conductive lands for each path may be selected to achieve a desired resonant frequency and a desired field contour.
The enclosure is preferably formed to break induced eddy currents and/or to shape the RF magnetic field patterns.
For some embodiments, a lid is mounted to at least one end of the coil. The lid may be conductive, non-conductive or segmented to be partially conductive. A plurality of sample spaces may also be formed in the dielectric at a selected portion of the enclosure, such portion being one of the side walls for an illustrative embodiment. The sample spaces extend at least part way into the dielectric from the side wall. Alternatively, the open center chamber or aperture of the coil may contain a dielectric which preferably fills such chamber and a plurality of sample spaces may penetrate such dielectric. At least a portion of at least selected ones of the paths may be formed as conductive tubes or coaxial tube conductors.
For embodiments having a close end wall, the closed end wall functions as an RF mirror, the end wall having a radial slotting pattern covering at least most of the wall for an embodiment where the electrical conductors for each wall are slotted.
For some embodiments, field applied to at least one of the electrically conducting paths causes an alternating magnetic field in the cavity and at least one aperture is formed in at least the electrical conductor through which magnetic field may be applied to an adjacent body. For some such embodiments, the coil is shaped to conform to the body being imaged, the surrounding walls including connected top and bottom walls, with the at least one aperture being formed in only the bottom wall. The coil may be flexible to conform to a surface of a body being imaged and the at least one apertures may be arranged to be adjacent the areas to be imaged of the body being imaged. Where the areas to be imaged are at least one projection on a body, such projection may extend into the cavity through an adjacent aperture. For such embodiments, the dielectric may be conformable to an outer surface of a projection extending into the cavity so as to minimize discontinuity between the projection and the dielectric. Where the coil is formed is a closed loop, an aperture may be formed in only one of an inner or outer wall of the coil. Apertures may be arranged to be adjacent areas to be imaged of a body being imaged.
Various features of the invention, such as the detuning mechanism, maybe employed independent of other features of the invention. Another potentially independent feature is the dielectric material filling the cavity being utilized to control the resonant frequency for one or more of the paths. In particular, the dielectric material may be different in different areas of the cavity so as to selectively shape the coil field. A mechanism may also be provided for controlling the dielectric fill of the cavity and thus the resonant frequency of the coil, for example the amount of fluid in the cavity being controlled where a fluid dielectric is employed. Acoustic damping material may also be provided as a fill for at least a portion of the cavity. A dielectric material may also be selectively positioned between the coil and a body to be imaged to control and/or shape the field applied to the body from the coil. The dielectric material preferably substantially fills the space between the coil and at least a selected area of the body, the dielectric constant of the dielectric substantially matching that of the body in such area. Where a selected area is the area to be imaged, the dielectric concentrates and directs the field to such area. In its broadest sense, the invention includes a conductive enclosure which is patterned to suppress low frequency currents and EMI noise.
In accordance with still another aspect of the invention, the RF coil includes a cavity formed by at least an inner and an outer conductor, a dielectric material filling the cavity and at least one sample space formed in the coil. The sample space may for example be formed in the dielectric material, projecting therein from a wall of the enclosure. For some embodiments, the coil is a transmission line stub, the inner and outer conductor of the coil being the inner and outer conductors of the transmission line stub, respectively. A conductive cap may short one end of the transmission line stub. The sample space is preferably located at a distal end of the stub, the sample space extending from such distal end into the dielectric material or a hollowed-out portion of the center conductor. The stub is tuned and matched so that maximum current, and therefore maximum RF field, occur at such distal end.
The foregoing and other objects, features and advantages of the invention will be apparent in the following more particular description of preferred embodiments of the invention as illustrated in the accompanying drawings.